The present invention pertains to the logging of earth formations by the use of logging instruments that have sources of high-intensity, high-energy photons and that are passed through a borehole traversing the earth formation of interest and, more specifically, to methods and apparatus for logging in which the photon source is capable of being pulsed and produces a burst of photons that has a continuous energy spectrum and in which the logging instrument includes at least one photon detector.
In order to avoid confusion, the term "photon" has been used throughout the present application to mean high frequency electromagnetic radiation regardless of how it originates. Therefore, the term is meant to include gamma rays, x-rays and Bremsstrahlung, each of which consist of high frequency electromagnetic radiation but are classified, in general, by the way in which they originate.
The usefulness of well logging instruments having photon sources and photon detectors for obtaining indications of earth formation density surrounding a borehole is well known, and the principles on which they operate are, similarly, well known to those skilled in the art. Generally, such density logging instruments take advantage of the Compton scattering effect, a phenomenon by which a photon is scattered in an interaction with an essentially free electron in the scattering medium. The photon scattering thus achieved can be detected and related to bulk density of the scattering medium as disclosed, for example, in U.S. pat. No. 3,321,625, issued May 23, 1967 to Wahl and assigned to the assignee of the present application or, more recently, as shown in the density computer described in U.S. Pat. No. 3,590,228, issued June 29, 1971 to Burke and also assigned to the assignee of the present invention.
Well logging instruments having photon sources and photon detectors are also useful for obtaining indications of earth formation lithology by making use of the photoelectric effect to derive an effective atomic number for an earth formation. Typical of present lithology logging techniques is that disclosed in U.S. pat. No. 3,864,569, issued Feb. 4, 1975 to Tittman and assigned to the assignee of the present application.
Previously, photon sources used for density and lithology logging purposes have included radioactive isotopes such as radium 226, cesium 137 or cobalt 60. Such isotopes are convenient to use, but they have several significant shortcomings: (1) The energy range of emitted photons is limited, thereby limiting the types of photon interactions that can be used for well logging purposes, (2) the maximum activity that can be safely and conveniently handled in field operations is limited to a few Curies, thereby limiting the precision of any media characteristic determination by limiting the numbers of photons returning to the borehole, (3) the isotopes are continuously emitting, so timing measurements are not possible, and (4) they emit photons uniformly in all directions, thereby complicating logging-instrument shielding problems.
As a result, present methods and apparatus for logging earth formations in which photons are emitted to penetrate the media surrounding a borehole and in which indications of photons resulting from the interaction of the emitted photons and the surrounding media are obtained to determine a characteristic of the media have had a limited depth of investigation; that is, they have been unable to determine media characteristics beyond a limited distance from the borehole into the media. Consequently, mudcake and other borehole distortions have significantly detracted from the accuracy of the desired media characteristic determination and have resulted in the expenditure of considerable effort in the well-logging industry toward the development of correction factors and charts to compensate for such borehole distorting effects. Furthermore, the types of photon interactions used for media characteristic determinations have been restricted, and those interactions that have been used have not provided the degree of precision desired.